Flatten Tree to Linked List

Given the root of a binary tree, flatten the tree into a Linked List:

• The Linked List should use the same TreeNode class where the right child pointer points to the next node in the list and the left child pointer is always null.
• The Linked List should be in the same order as a pre-order traversal of the binary tree.

Class TreeNode:
    val (int)
    left (TreeNode)
    right (TreeNode)

Input Format:

First Parameter - TreeNode root

Output Format:

Return the TreeNode

Example 1:

Input:
    root = [1 2 5 3 4 null 6]
Output:
    [1 null 2 null 3 null 4 null 5 null 6]

Example 2:

Input:
    root = [4]
Output:
    [4]

Constraints:

• The number of nodes in the tree is in range [1,2000].
• -100 <= node.val <= 100
• Expected Time Complexity: O(N).
• Expected Space Complexity: O(1).